A major aim is to continue studies on the in vivo roles of eukaryotic DNA topoisomerases using yeast topoisomerase mutants and genes isolated previously. Five separate projects are proposed: 1) to determine the role of topoisomerases in ribosomal RNA synthesis, 2) to learn why overproduction of DNA topoisomerase II is deleterious to yeast, 3) to clone human topoisomerase I and II genes by direct selection in yeast. A human cDNA library will be placed in a yeast expression vector plasmid. The DNA will be used to transform a yeast mutant deficient in topoisomerase I and II; rare transformants which have gained topoisomerase I or II activity will be selected by their more rapid growth. DNA topoisomerases are known to be the targets of a wide variety of anti-cancer drugs. Isolation of their genes will allow overproduction of the enzymes, which will be useful for testing new derivatives of the known drugs, 4) to determine if the in vivo target of the drug novobiocin in yeast is topoisomerase II or some other enzyme, and 5) to determine if RNA polymerase II, the enzyme responsible for mRNA synthesis, has topoisomerase activity inherent in it, distinct from the known topoisomerases I and II. Another major aim is to find a yeast mutant deficient in histone acetyl transferase and to use the mutant to determine the role of histone acetylation in chromatin structure and function. The approach will be the same as was used successfully to identify yeast topoisomerase mutants, namely, to screen a collection of heavily mutagenized temperature-sensitive mutants by direct enzymatic assay. Once a mutant is identified, its phenotypes will be studied to learn about the in vivo role of the enzyme.